1. Field of the Invention
The present invention relates to a radio communications system, a transmitter, a receiver, a radio transmission method, a radio reception method and a computer program therefor.
2. Description of Related Art
FIGS. 8A and 8B schematically show a radio communications system for communicating information using impulse signals. A transmitter 101 uses an impulse signal sequence comprising consecutive impulses having a predetermined impulse period as shown in FIG. 8B to communicate information to a receiver 102. Examples of such an information communicating method using an impulse signal sequence include Ultra Wide Band (UWB) communications systems and the like. UWB communications systems communicate information by transmitting impulse signals at predetermined repetition periods. In other words, a base band communication is performed using a signal comprising pulse sequences having a very narrow pulse width (e.g., 1 ns (nanosecond) or less). The occupied bandwidth is on the order of GHz and is such that the value obtained by dividing the occupied bandwidth by the center frequency thereof (e.g., 1 GHz to 10 GHz) is approximately 1. The above-mentioned bandwidth is much greater than bandwidths ordinarily used for wireless LAN communications employing W-CDMA®, CDMA2000®, SS (Spread Spectrum), OFDM (Orthogonal Frequency Division Multiplexing) technologies and the like.
UWB communications schemes are characteristic in that, due to the low power spectral density properties thereof, they hardly interfere with other radio communications systems. Therefore, there are expectations for UWB communications as a technology capable of overlaying frequency bands used by existing radio communications systems. Further, because ultra wide bandwidths are used, UWB communications is viewed as a promising technology which could realize ultra high-speed radio communications on the order of 100 Mbps in its application to personal area network (PAN) technology.
In communicating information using the impulse signals mentioned above, the amplitude of each of the impulses constituting the impulse signals are handled by a transmitter and a receiver as being constant.
FIGS. 9A through 9I explain a UWB communications scheme which is an example of a method of communicating information using an impulse signal sequence. In UWB communications, 1 bit of information to be communicated from a transmitting end is transmitted using several impulses. Namely, 1 bit of information 501 (FIG. 9A) is directly spread using a predetermined spreading code 502 (FIG. 9B), and is thereafter converted into a spread signal 503 (FIG. 9C). Corresponding to its value of 0 or 1, the spread signal 503 is converted into a very fine impulse sequence and is transmitted as a transmission signal 504 by way of an antenna. FIG. 9E is an enlarged view of a portion 504-1 showing the first few pulses of the transmission signal 504. As is apparent therefrom, in UWB schemes, the amplitude of each transmitted pulse is uniform.
On a receiving end, a signal, which is the transmission signal 504 onto which noise is superimposed, is received as a received signal 505 (FIG. 9F). The received signal 505 is despreaded using a predetermined despreading code 506 (FIG. 9F). In other words, the received signal 505 is detected with a correlator, and a correlator output 507 (FIG. 9H) is obtained. A spreading code is integrated in the correlator output 507, and the correlator output 507 becomes an integrated signal 508 (FIG. 9I).
Problems associated with radio communications systems using conventional impulse signals are explained below. FIG. 10 shows an arrangement of a radio communications system in which each of receivers 202 and 204 receives a signal from a plurality of transmitters 201 and 203. In this case, when a condition arises whereby the transmitters 201 and 203 happen to start transmission at an identical pulse position, each of the receivers 202 and 204 simultaneously receives pulses from the transmitters 201 and 203. If there is a large power difference between the pulses from the desired (relevant) transmitter and the pulses from an interfering transmitter, the pulses with greater power is received due to the “Capture Effect.” On the other hand, when the power difference between the desired transmitter and the interfering transmitter is small, neither of the pulses are received and both pulses are lost.
Especially in a PAN, because a base station is often absent, it is rare for transmission power control at the transmitter to be performed. Therefore, the closer a transmitter is to a receiver, the greater the power of the received pulse becomes. Thus, when the interfering transmitter is at a position closer to the receiver than the desired transmitter is, due to the capture effect mentioned above, only interfering pulses are received.